Heretofore, a piezoelectric element has been used for an ignition element, an ultrasonic vibrator, an ultrasonic inspector, a medical ultrasonic probe, a fish finder, a frequency filter, an acoustic element, a piezoelectric actuator, or the like. The ultrasonic vibrator is a device which drives or vibrates a piezoelectric element by an electric field having a high frequency in an ultrasonic range, such as an ultrasonic motor or a washing vibrator.
The ultrasonic inspector transmits an ultrasonic vibration obtained in the same manner as the ultrasonic vibrator to a material to be inspected, such as an iron plate, to find defects such as cracks in the inspected material from information of echoes reflected back from the cracks.
By applying the same principle to organisms, the medical ultrasonic probe inspects tissues of humans.
The fish finder transmits an ultrasonic wave into water to search fish by using information reflected back from fishes.
The actuator accurately shows microscopic displacements in orders of micrometers or under micrometers by applying a voltage, and application to sounds like buzzers, precise control of a flow rate of a pump, a valve or the like, autotracking of a VTR head, autofocus, an equipment to accurately determine a placement of a mechanical cutting tool in a range of micrometers, an equipment for producing a semiconductor to determine a microscopic displacement, or the like, has been rapidly developed these days.
As a material for these piezoelectric elements, a PZT(PbZr.sub.x Ti.sub.y O.sub.3 :x+y=1,x.gtoreq.0,y.gtoreq.0)type material which contains lead, zirconium, titanium or the like and is an ABO.sub.3 type 25 perovskite compound having a divalent ion of lead or the like on the A site and a tetravalent ion of Zr, Ti or the like on the B site, is generally used. Specifically, a solid solution of the above PZT and a complex perovskite compound wherein the B site is on the average tetravalent, such as Pb(Ni.sub.1/2 W.sub.1/2)O.sub.3, Pb(Co.sub.1/3 Ni.sub.2/3)O.sub.3,Pb(Ni.sub.1/3 Nb.sub.2/3)O.sub.3 or the like, is already known.
The piezoelectric material is selected by searching a composition wherein a dielectric constant, an electromechanical coupling constant, a piezoelectric strain constant or a mechanical displacement caused by inducing an electric field is large.
It is clarified that the dielectric constant, the electromechanical coupling constant, the piezoelectric strain constant or the mechanical displacement caused by inducing an electric field generally indicates a maximum value when the above PZT type compound is in a state around a crystal phase boundary of a rhombohedral and a tetragonal (this is referred to as morphotropic phase boundary, and will be simply referred to as .left brkt-top.MPB.right brkt-bot. hereinafter), and the material is developed by searching the MPB.
For example, as a result of crystal phase identification by an X-ray diffraction measurement, it is crystallographically clarified that a crystal system of a pure PZT near room temperature is tetragonal when a ratio of constitutive elements on the B site of a perovskite structure, i.e. a molar ratio of Zr/Ti is at least 0/1 and is less than 0.53/0.47, that it is rhombohedral when Zr/Ti exceeds 0.53/0.47 and is at most 0.90/0.10, and that a crystal phase boundary of tetragonal/rhombohedral, i.e. MPB exists around Zr/Ti=0.53/0.47. Further, it is known that the piezoelectric properties such as a dielectric constant, an electromechanical coupling constant and a piezoelectric strain constant become maximum around the MPB composition.
Heretofore, the piezoelectric element has been used around room temperature (for example, 10-30.degree. C.), and therefore it has been sufficient to search a composition having excellent properties such as a large dielectric constant, a large electromechanical coupling constant, a large piezoelectric strain constant and a large displacement caused by inducing an electric field in such a temperature range. Accordingly, a method for driving a piezoelectric element suitable for use in a wide temperature range, has not been developed yet.
However, when the temperature range to be used is wide such as from room temperature to 100.degree. C. or to a higher temperature (for example, 150.degree. C.), from -50.degree. C. to room temperature, or from -50.degree. C. to 100.degree. C. or to a higher temperature (for example, 150.degree. C.), a displacement caused by inducing an electric field is as follows. A displacement amount caused by a voltage applied around room temperature remarkably changes with temperature. Accordingly, the displacement amount is remarkably small in a low temperature range, and on the contrary, the displacement amount remarkably increases in a high temperature range. Therefore, it was difficult to obtain a desired displacement in the whole range of temperature to be used.
Accordingly, to stably obtain a desired certain displacement in a wide temperature range, a method to change a voltage to be applied depending on driving temperatures, for example, application of a high voltage in a low temperature range and application of a low voltage in a high temperature range, is considered, but an expensive driving device is necessary therefor.
Next, a resistance related to difficulty of depolarization under an electric field in a direction opposite to polarization of a piezoelectric material, is explained. FIG. 1 explains electric coercive field which is a measure for easiness of changing a polarized state, and shows a relation between polarization (D) and an electric field (E) of a piezoelectric material. As clear from FIG. 1, when to a polarizated piezoelectric material {state(0)}, an electric field in a direction opposite to the polarization direction is applied, an intensity of polarization of the piezoelectric material decreases. This is because the direction changes to the opposite direction, i.e. to the direction of the electric field thus applied, and depolarization occurs. When the electric field in the opposite direction is remarkably intensified, the polarization in the opposite direction is saturated via {state(1)} to be in {state(2)} (see FIG. 1). During it, there is a state when an intensity of polarization of the piezoelectric material become 0, and in this state, the polarization of the piezoelectric material is directed in various directions to cause the state polarization=1. Such an intensity of an electric field when polarization become 0 (Ec) is electric coercive field. A material having a large electric coercive field is resistant to an electric field in a direction opposite to a polarization treatment, and is difficult to be depolarized.
Generally, a material to be used for a piezoelectric actuator an ultrasonic inspector, a medical probe or the like is what is called a soft type material, and therefore, the above electric coercive field is small and the electric coercive field becomes much smaller at a high temperature. Accordingly, when it is driven at a high temperature such as over 80.degree. C., there were problems in durability of displacement as follows. That is, when it is maintained at a high temperature for a long time in a state wherein no voltage is applied, it is depolarized, or when an electric field in a direction opposite to polarization is applied, the piezoelectric element becomes to be depolarized more easily.
A dielectric constant of a piezoelectric element generally increases with temperature. When a piezoelectric material is driven alternately in the electric field, the dielectric constant becomes higher because of a higher temperature. Therefore, dielectric loss increases, it becomes to generate heat more easily, and it is depolarized. That is, these problems in durability existed.
An object of the present invention is to solve the above problems, to have a displacement or a dielectric constant of a piezoelectric element less dependent on temperature, and to provide a piezoelectric element wherein decrease of electric coercive field at a high temperature is suppressed and a method for a driving same.